It is known to represent an image by a plurality of pixels arranged as an array of rows and columns of pixels. Typically the pixels are considered to be in a sequential order form left to right and top to bottom. A pixel represents the image value of a tiny portion of the image 2 (FIG. 1). A pixel is represented by a digital data field representing the image value (for example, the color and brightness) of the pixel. FIG. 1 is a diagram illustrating the correspondence between pixels 4 representing an image 2 and a data matrix 6 containing data fields 5 representing the pixels 4 in the image 2. In FIG. 1, an image 2 is represented by rows and columns of respective pixels 4. In FIG. 1, a pixel is represented by a composite 24 bit digital data field, containing an 8 bit data component representing the red content of the pixel, an 8 bit data component representing the blue content of the pixel, and an 8 bit data component representing the green content of the pixels. The digital data fields 5, representing the pixels 4 in the image 2, are stored in a data matrix 6 in a memory. The 24 bit data field representing the top left pixel 8a is stored at location 9a in the data matrix 6; the 24 bit data field representing the pixel 8b is stored at location 9b in the data matrix 6; the 24 bit data field representing the pixel 8c is stored at location 9c in the data matrix 6, and so forth. While the data matrix 6 is displayed in FIG. 1 as rectangular, and with locations of the pixels 4 representative data fields 5 corresponding to the location of the pixels 4 in the image 2, one skilled in the art understands that FIG. 1 is schematic only and any arrangement of the pixels 4 representative data fields 5 in a data matrix 6 may be used, provided a correspondence between data field 5 and pixels 4 is maintained.
The Digital Visual Interface (DVI) standard is a video interlace standard designed to provide relatively high visual quality images on digital display devices. The DVI interface uses a digital protocol in which the desired color of respective pixels is transmitted as binary data. When a display device is driven at its native resolution, it will read each pixel color representative binary number and apply that color to the appropriate pixel on the display. In this way, each pixel in the output buffer of the source device corresponds directly to one pixel in the display device.
The data format used by DVI is a serial format. A single DVI link consists of four twisted pairs of wires transmitting respective serial signals representing red, green, blue color components of sequential pixels, and corresponding clock signals. As described above with respect to FIG. 1, 24 bits per pixel are transmitted: one color component representative data field of 8 bits serially through the corresponding twisted pair. The picture is transmitted pixel by pixel from left to right in each line; and line by line, with blanking intervals between each line, from top to bottom for each frame. That is, in FIG. 1, the 24 bit data field representing the pixel 9a, is transmitted; followed by the 24 bit data field representing the pixel 9b; followed by the 24 bit data field representing the pixel 9c, and so forth, as represented by the arrow pointing to the right. When the pixels in the top row have been transmitted, followed by data representing blanking and synchronization, the pixel representative data in the second row are transmitted in the same order, followed by the pixel representative data in the third row, and so forth, as represented by the arrow pointing downward. This data stream is received by a display device which reproduces the image represented by the respective pixel representative data.
With a single DVI link, the largest resolution possible at 60 Hz is 2.75 megapixels (including blanking intervals). For practical purposes, this allows a maximum screen resolution at 60 Hz of 1915×1436 pixels (standard 4:3 ratio), 1854×1483 pixels (5:4 ratio) or 2098×1311 (widescreen 8:5 ratio). The DVI format also has provision for a second link, containing another set of red, green, and blue twisted pairs which operate in conjunction with the twisted pair. When more resolution and/or bandwidth is required than is possible with a single link, the second link is enabled, and alternate pixels are transmitted concurrently on the respective sets of red, green and blue twisted pairs in the first and second links, allowing resolutions up to 4 megapixels at 60 Hz. The DVI specification mandates a fixed single link maximum pixel clock frequency of 165 MHz, where display modes that require less than this use the single link mode, and those that require more switch to the dual link mode. The data link pairs carry binary data at ten times the pixel clock reference frequency, for a maximum data rate of 1.65 Gbit/s×3 data pairs for a single DVI link.
In typical video boards incorporating a DVI format, two separate display devices may be connected to a single video board. In cases where more than two display devices are desired, an additional video board is added to the system. This permits two more display devices to be connected. However, an additional video board adds expense to the computer system, and may take up high speed connectors on the computer system mother board. Such connectors are typically scarce. Because of the expense and use of scarce resources caused by the use of two DVI video boards, it is desirable to be able to display two or more less-than-full resolution images on a single DVI signal.